Editing Avalanche (section)

=== Terrain ===
Avalanche formation requires a slope shallow enough for snow to accumulate but steep enough for the snow to accelerate once set in motion by the combination of mechanical failure (of the snowpack) and gravity. The angle of the slope that can hold snow, called the [[angle of repose]], depends on a variety of factors, such as crystal form and moisture content. Some forms of drier and colder snow will only stick to shallower slopes, while wet and warm snow can bond to very steep surfaces. In coastal mountains, such as the [[Cordillera del Paine]] region of [[Patagonia]], deep snowpacks collect on vertical and even overhanging rock faces. The slope angle that can allow moving snow to accelerate depends on a variety of factors such as the snow's shear strength (which is itself dependent upon crystal form) and the configuration of layers and inter-layer interfaces.{{fact|date=January 2024}}

The snowpack on slopes with sunny exposures is strongly influenced by [[Sunlight|sunshine]]. Diurnal cycles of thawing and refreezing can stabilize the snowpack by promoting settlement. Strong freeze-thaw cycles result in the formation of surface crusts during the night and of unstable surface snow during the day. Slopes in the lee of a ridge or of another wind obstacle accumulate more snow and are more likely to include pockets of deep snow, [[wind slabs]], and [[cornice (climbing)|cornices]], all of which, when disturbed, may result in avalanche formation. Conversely, the snowpack on a windward slope is often much shallower than on a lee slope.<ref>{{Cite web |title=Avalanche Safety Guidelines |url=https://www.ehss.vt.edu/uploaded_docs/201006231613220.Avalanche_Safety_Guideline.pdf |access-date=April 10, 2024 |website=www.ehss.vt.edu}}</ref>

[[File:Avalanche path 7271.JPG|thumb|left|Avalanche path with {{convert|800|m|ft}} vertical fall in the [[Glacier Peak Wilderness]], [[Washington (state)|Washington state]]. Avalanche paths in alpine terrain may be poorly defined because of limited vegetation. Below tree line, avalanche paths are often delineated by vegetative trim lines created by past avalanches. The start zone is visible near the top of the image, the track is in the middle of the image and clearly denoted by vegetative trimlines, and the runout zone is shown at the bottom of the image. One possible timeline is as follows: an avalanche forms in the start zone near the ridge, and then descends the track, until coming to rest in the runout zone.]]
Avalanches and avalanche paths share common elements: a start zone where the avalanche originates, a track along which the avalanche flows, and a runout zone where the avalanche comes to rest. The debris deposit is the accumulated mass of the avalanched snow once it has come to rest in the run-out zone. For the image at left, many small avalanches form in this avalanche path every year, but most of these avalanches do not run the full vertical or horizontal length of the path. The frequency with which avalanches form in a given area is known as the [[return period]].<ref>{{Cite web |title=Return period calculated for study snow avalanche paths using the existing method |url=https://www.researchgate.net/figure/Return-period-calculated-for-study-snow-avalanche-paths-using-the-existing-method_fig4_327258239 |access-date=April 10, 2024 |website=www.researchgate.net}}</ref>

The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally [[Convex function|convex]] slopes are less stable than [[concave function|concave]] slopes because of the disparity between the [[tensile strength]] of snow layers and their [[compressive strength]]. The composition and structure of the ground surface beneath the snowpack influences the stability of the snowpack, either being a source of strength or weakness. Avalanches are unlikely to form in very thick forests, but boulders and sparsely distributed vegetation can create weak areas deep within the snowpack through the formation of strong temperature gradients. Full-depth avalanches (avalanches that sweep a slope virtually clean of snow cover) are more common on slopes with smooth ground, such as grass or rock slabs.<ref>{{Cite web |title=Glossary |url=https://avalanche.ca/glossary |access-date=2024-04-10 |website=avalanche.ca |language=en}}</ref>

Generally speaking, avalanches follow drainages down-slope, frequently sharing drainage features with summertime watersheds. At and below [[tree line]], avalanche paths through drainages are well defined by vegetation boundaries called [[trim line]]s, which occur where avalanches have removed trees and prevented regrowth of large vegetation. Engineered drainages, such as the [[avalanche dam on Mount Stephen in Kicking Horse Pass]], have been constructed to protect people and property by redirecting the flow of avalanches. Deep debris deposits from avalanches will collect in catchments at the terminus of a run out, such as gullies and river beds.

Slopes flatter than 25&nbsp;degrees or steeper than 60&nbsp;degrees typically have a lower incidence of avalanches. Human-triggered avalanches have the greatest incidence when the snow's [[angle of repose]] is between 35 and 45&nbsp;degrees; the critical angle,<ref name=":1" /> the angle at which human-triggered avalanches are most frequent, is 38&nbsp;degrees. When the incidence of human triggered avalanches is normalized by the rates of recreational use, however, hazard increases uniformly with slope angle, and no significant difference in hazard for a given exposure direction can be found.<ref name="Pascal Hageli et al">{{Cite web |last=Hageli |first=Pascal |display-authors=etal |title=AVISUALANCHE – SELECTED PUBLICATIONS |url=http://www.avisualanche.ca/publications.html |website=www.avisualanche.ca}}</ref> The rule of thumb is: ''A slope that is flat enough to hold snow but steep enough to ski has the potential to generate an avalanche, regardless of the angle.''{{fact|date=January 2024}}